This application relates to modulators for wireless transmitters, and more particularly to modulators for wireless transmitters consistent with the EDGE standard.
Wireless handsets or terminals communicate with each other through the air using base stations or cell antennas as intermediaries. Each handset or terminal includes a transmitter and a receiver. Transmitters receive binary data and convert this data to symbols, forming a signal that is transmitted. Receivers receive the transmitted symbols, and convert them to binary data. The base stations are capable of receiving and transmitting several signals simultaneously.
The frequency at which a handset transmits symbols is determined by a clock signal that is generated locally, that is, in the handset. This clock signal is typically generated using a crystal or other periodic clock source. These crystals are highly accurate, for example, they may be accurate to within a few parts per million. But a base station has its own locally generated clock source, which is, typically, extremely accurate, and does not necessarily match the frequency of the clock signal in a particular handset. Also, to reduce costs of the handsets, it is desirable to use rower costs crystals having higher tolerances, leading to larger frequency differences between a handset and a base station.
Accordingly, a frequency correction may be used to correct for differences in frequency between the handset crystal and the base station clock. A base station typically communicates with several handsets. Thus it is desirable for the frequency correction to occur at the handset—it would be very difficult for the base station to adjust to several handsets simultaneously. Instead, a frequency offset may be determined by comparing the frequency of symbols received from a base station to the handset clock signal. This frequency offset may then be used to correct the frequency of symbols transmitted from the handset to the base station.
Also, the EDGE standard provides for a 3π/8 phase shift to each transmitted symbol. This phase shift reduces zero crossings seen at the transmitter power amplifier, and the fluctuations in the output envelope are reduced.
Thus, it is desirable to have a modulator that provides frequency correction, a 3π/8 phase shift, and converts symbols to I and Q component values for transmission.